1. Field of the Invention
The present invention relates to an improved element for the chemical analysis of liquids. More particularly it concerns multilayer elements that provide a means for determining the presence and/or concentration of a substance in a liquid by effecting the release or formation of a detectable species, such as a dye, within the element in quantities proportionate to the concentration of the substance in the liquid being analyzed. The present invention provides a means for inhibiting the migration or wandering of a substantial portion of this detectable species to areas or layers of the element in which the presence of the detectable species cannot easily be determined, either quantitatively or qualitatively.
2. Description of Related Art
It is often desirable or necessary to determine the presence and/or concentration of certain substances in liquids such as water, foodstuffs, and biological liquids. A variety of devices and methods have been employed for such analyses.
Various inexpensive elements have been devised to facilitate rapid and convenient analyses under other than controlled laboratory conditions. Such elements often include a reagent for the analyte (the term analyte referring to the substance being analyzed for in the liquid sample). This reagent, upon contact with the analyte, causes the formation of a dye or brings about some other detectable change to indicate the presence of analyte in the liquid sample. One example of such an element is a pH test strip that comprises a paper or other absorbent material impregnated with an appropriate reagent or reagents. Simple elements of this type are most often employed when it is only necessary to make a quick visual determination of the presence of the analyte qualitatively or at best semi-quantitatively.
More sophisticated elements are available for quantitative diagnostic analyses of biological liquids like blood or urine. When a liquid sample containing the analyte is brought into contact with these elements, they form the dye or other detectable change consistently and uniformly within the element in proportion to the concentration of the analyte in the liquid sample. Analyte concentration can then be determined, for example, by spectrophotometric measurement of the optical density of the dye formed in the element.
Elements of this type are described in U.S. Pat. No. 3,992,158, issued Nov. 16, 1976. These elements can consist of two or more desirably discrete layers that are superposed and in substantially continuous intimate contact with adjacent layers. One such multilayer element comprises a support layer having a reagent layer and an outermost spreading layer coated upon it. In this multilayer element, the spreading layer serves as a liquid sample permeable receiving and metering layer. That is, the liquid sample to be analyzed is placed on the spreading layer, which absorbs and transfers the liquid to the reagent layer. Preferably, as described in U.S. Pat. No. 3,992,158, the spreading layer is isotropically porous and transfers a uniform concentration (as measured across a per unit cross-sectional area of the spreading layer) of the analyte contained in the liquid sample to the underlying reagent layer. The reagent layer has certain reagents uniformly distributed therein. A detectable species such as a dye is formed within the reagent layer in an amount proportional to the concentration of analyte in the liquid. Typically, the reagent and support layers are radiation-transmissive so that a spectrophotometric measurement of the optical density of the dye formed in the reagent layer can be made with the element remaining intact. Additionally, the spreading layer may comprise a blushed polymer and a pigment to provide both uniform transfer of the liquid sample to the reagent layer and an opaque, reflective surface above the reagent layer to aid in a measurement of reflection density of the dye. With this element, however, some of the dye formed in the reagent layer may migrate or wander into the opaque spreading layer where it would not be detected during the dye-density measurement, thereby reducing the sensitivity and the accuracy of the analysis.
Related elements are described in U.S. Pat. No. 4,042,335, of Clement, issued Aug. 16, 1977. A registration layer and an opaque or radiation-blocking layer are coated between the support layer and the reagent layer. During the analysis, a significant portion of the detectable species, e.g., a dye, formed in the reagent layer will diffuse through the radiation-blocking layer and into the registration layer, where the dye density will be measured. A mordant for the dye can be included in the registration layer to insure that the dye that has diffused into this layer will be fixed there for easy detection and will not be allowed to diffuse or migrate out of the registration layer. Elements such as this are suggested for use where it would not otherwise be practical to reliably measure the dye density within the reagent layer itself, for example, in analytical elements where other reagents and reaction products within the reagent layer also provide density, thus preventing any accurate spectrophotometric measurement of the optical density in this layer of only the dye. Such an element can provide a reliable analysis. However, it is obvious that a significant portion of the dye formed during the analysis can remain in the reagent layer or migrate into and remain in the radiation-blocking layer. The sensitivity and accuracy of the analytical element are thereby reduced, because the analyte-concentration determination must depend upon the measurement of the density of a smaller amount of dye than that which was actually formed.
Other elements as described in U.S. Pat. No. 3,585,112 and U.S. Pat. No. 3,917,453 disclose means for overcoming these problems. Both of these patents suggest the use of mordants in the reaction zone or layer to provide a degree of immobility to the indicator dye formed. These elements, like others of the prior art, however, are susceptible to the additional problem of the mordant interfering with the formation of the dye or interfering with any prerequisite reactions leading to the formation of the dye. Such interference can make the analysis completely unreliable.
Accordingly, it is desirable to provide an analytical element that has all of the advantages of the elements described above, i.e., ease of use, low cost and quantitative results; and that also overcomes the problems inherent in prior art elements, such as reduced sensitivity and accuracy of results caused by (a) migration of detectable species into porous radiation-blocking layers and (b) interference with the formation or release of the detectable species by mordants used to inhibit such migrations.